Flame-resistant fiber is so excellent in heat resistance and flame retardance as to be widely utilized, for example, for spatter sheets for protecting the human body from high-heat iron powder and weld spark, which scatter in welding operation, and fire-resistant heat insulators for aircrafts, leading to an increasing demand in the fields.
Flame-resistant fiber is important also as intermediate raw materials for obtaining carbon fiber. The carbon fiber has such mechanical, chemical properties and lightweight properties as to be widely used for various uses, for example, materials for aviation and space such as aircrafts and rockets, and sporting goods such as tennis rackets, golf shafts and fishing rods, and to be going to be used also in fields for transport machines such as vessels and automobiles. In recent years, the carbon fiber has so high electrical conductivity and heat radiation as to be strongly required for application to electronic equipment parts such as portable telephones and personal computer cabinets, and electrodes of fuel cells.
The carbon fiber is generally obtained by a method of carbonizing flame-resistant fiber by heating at high temperature in an inert gas such as nitrogen. With regard to flame-resistant fiber, for example, polyacrylonitrile (PAN)-based flame-resistant fiber is obtained by making PAN-based precursor fiber flame-resistant (cyclization reaction and oxidation reaction of PAN) at a high temperature of 200 to 300° C. in the air.
However, this reaction for making flame-resistant is an exothermic reaction and a reaction in a fibrous form, namely, a solid-phase state. Therefore, long-time treatment is required for temperature control, and the degree of fineness of PAN-based precursor fiber needs to be limited to fine size below specific value for finishing making flame-resistant within desired time. Thus, the presently known process of making flame-resistant is regarded with difficulty as a sufficiently efficient process.
With regard to flame-resistant products, it is substantially difficult to obtain flame-resistant formed products in the form except fiber, such as plane shapes of sheet and film and various cubic shapes, due to the difficulty of heat removal for the reason that the reaction for making flame-resistant is an exothermic reaction as described above. Accordingly, flame-resistant formed products are limited to fibreform products, and in the present circumstances, plane sheets are manufactured by making such fibreform products into fabrics.
When flame-resistant fiber with optional degree of fineness and flame-resistant products except fibreform products (flame-resistant formed products), such as sheet-like products and cubic molded products, are obtained, the use of flame-resistant formed products is markedly extended. In addition, the appropriateness of manufacturing conditions and carbonizing conditions thereof allows carbon fiber with optional degree of fineness and carbon products except fibreform products (carbon product), such as a carbon product group of sheet-like carbon and cubic carbon molded products, and extends uses thereof. The improvement of yield while maintaining high physical properties of carbon products brings advantages in costs.
Dissolution by solvent has been studied as a method for solving the above technical problem.
For example, a technique is disclosed such that acrylonitrile polymer powder is heated in inert atmosphere until the density becomes 1.20 g/cm3 or more, and thereafter dissolved in solvent and fiberized into a fibriform product, which is heat-treated (for example, refer to Patent Document No. 1).
However, the problem is that viscosity change with time of solution is so great as to frequently cause thread breakage by reason of using acrylonitrile polymer powder with flame resistance less performed. A device made of special materials having corrosion resistance needs to be used by reason of using as solvent strongly acidic solvents such as sulfuric acid and nitric acid for easily decomposing general organic polymers, leading to impractical costs.
A method is proposed such that heat-treated acrylonitrile polymer powder and not heat-treated acrylonitrile polymer powder are mixed and similarly dissolved in acidic solvent (for example, refer to Patent Document No. 2); however, the problem is still not solved on allowing corrosion resistance to a device as described above and instability of solution.
In addition, the conversion of polyacrylonitrile to a polymer having cyclization chemical structure by heat-treating dimethylformamide solution of polyacrylonitrile is disclosed (for example, refer to Non-Patent Document No.1); however, the polymer solution is dilute concentration of 0.5% and so low in viscosity as to be substantially difficult in forming into fiber, and a rise in concentration thereof causes the polymer to be deposited and incapable of being used as solution.
On the other hand, a solution in which polyacrylonitrile is denatured with a primary amine is disclosed (for example, refer to Non-Patent Document No.2); however, the solution is such as to impart hydrophilic property to polyacrylonitrile itself with flame resistance less performed, and totally differs in technical ideas from a flame-resistant polymer-containing solution.
A technique is disclosed such that the yield can be improved together with high physical properties in a conversion example from flame-resistant fiber to carbon fiber on the special carbonizing conditions (for example, refer to Patent Document No.3); however, compatibility therebetween in an easier method has been demanded.    [Patent Document No. 1] Japanese Patent Publication No.63-14093B    [Patent Document No. 2] Japanese Patent Publication No.62-57723B    [Patent Document No. 3] Japanese Patent Publication No. 26365093B    [Non-Patent Document No.1] “Polymer Science (USSR)”, 1968, Vol. 10, page 1537    [Non-Patent Document No.2] “Journal of Polymer Science, Part A: Polymer Chemistry”, 1990, Vol. 28, page 1623